physicsofultrasound - A. Producing an Ultrasound Image...

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A. Producing an Ultrasound Image Sound is a longitudinal mechanical wave that requires a medium to travel through. A sound wave travels by compressing and rarefacting matter. Depending on the matter the material it is passing through – the wave will travel at different velocities. Ultrasound probes emit and receive this energy as waves to form pictures. A cycle is a repetitive periodic oscillation. This is just the period from where a sine wave increases until a maximum to where the same wave reaches zero after its minimum value has been reached. The number of cycles per second is called the frequency. A frequency of about 2.5-20 MHz is used for diagnostic ultrasound imaging. Lower frequencies (2.5-3.5 MHz) are used for deeper structures like the aorta, because it offers greater penetration, but comes with less resolution. Higher frequencies (5-10 MHz) are used for shallow structures near the surface of the skin like abscesses. High frequency offers greater resolution but comes with less penetration through layers of the body. A wavelength is a length of one complete wave cycle. The height of a sine wave above its average value is called the amplitude. In order to produce an ultrasound image we must take all of these factors into account. A transducer measures the time from emission to the return of an echoed sound wave pulse. Propagation velocity (c) is the velocity that a sound has when transmitted through a medium. It is also the product of the frequency (f) times the wavelength (). Propagation velocity is increased by increasing stiffness, and reduced by increasing density. Soft tissue has an average of about 1,540 m/sec with bone at about 4,080 m/sec. Each material a sound wave travels through has its own impedance (z) value that is a product of its density and propagation velocity. Differences in acoustic impedance create reflective interfaces that echo the ultrasound waves back at the probe that emitted them. Changes in velocity can be modeled by Snell’s law, in which sound transmitted through two different surfaces will have different velocities due to the differences in angels of incidence and refraction, as well as the refractive index of the material. -
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This note was uploaded on 02/01/2011 for the course BME 314 taught by Professor Frey during the Spring '08 term at University of Texas at Austin.

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physicsofultrasound - A. Producing an Ultrasound Image...

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